The present invention relates to an electric injection molding machine for injecting molten resin into a mold to obtain a molding.
An electric injection molding machine according to the related art will be explained with reference to FIG. 3 disclosed in Japanese Patent Application Publication (KOKAI) No.Hei.4-334430 hereunder.
In FIG. 3, an electric injection molding machine that pours resin 3 into a mold 1 to obtain an object having a predetermined shape has an injection molding machine 10 having a screw 7 in a cylinder 5, a pressure sensor 12 for sensing the pressure in the cylinder 5 based on the pressure applied to the screw 7, a servo motor 14 coupled/fixed to the screw 7, a position/speed sensor 16 for sensing the position and the speed based on the rotation angle of the servo motor 14, a current sensor 18 for sensing the current flowing through the servo motor 14, and a control portion 20 for driving/controlling the servo motor 14 based on sensed values of the pressure sensor 12, the position/speed sensor 16, and the current sensor 18 and command values.
The control portion 20 has a pressure subtracter 22 for calculating a pressure deviation between a pressure command Pr of the screw 7 and a pressure sensed value Pa of the pressure sensor 12, a pressure amplifier 24 for getting a speed command Nr by multiplying the pressure deviation by a dwell pressure control gain, a speed subtracter 26 for calculating a speed deviation between a speed command Nr and a speed sensed value Na, a speed amplifier 28 for getting a current command Ir by multiplying the speed deviation by a speed control gain, a current subtracter 30 for calculating a current deviation between the current command Ir and a current sensed value Ia, and a current amplifier 32 for getting a power command Wr by multiplying the current deviation by a current control gain.
An operation of the electric injection molding machine constructed as above will be explained with reference to FIG. 3 hereunder.
First, assuming that an injection mode for injecting the molten resin 3 into the mold 1 is completed and then the operation is shifted to a dwell pressure mode for holding the pressure in the mold 1 at a predetermined value.
In such dwell pressure mode, the pressure command Pr in the mold 1 and a position command (not shown) of the screw 7 are input, the pressure subtracter 22 calculates the pressure deviation between the pressure command Pr and the pressure sensed value Pa from the pressure sensor 12, and the pressure amplifier 24 calculates the speed command Nr by multiplying the pressure deviation by the dwell pressure control gain and applies the speed command Nr to the speed subtracter 26.
The speed subtracter 26 calculates the speed deviation between the speed command Nr and the speed sensed value Na of the position/speed sensor 16. The speed amplifier 28 obtains the current command Ir by multiplying the speed deviation by the speed control gain. The current subtracter 30 calculates the current deviation between the current command Ir and the current sensed value Ia of the current sensor 18. The current amplifier 32 obtains the power command Wr by multiplying the current deviation by the current control gain and drives the servo motor 14 to proceed the screw 7 until the pressure applied to the shaft of the screw 7 become equal to the command value, so that the pressure is controlled at a predetermined value. Then, the operation is shifted to a back-pressure mode in which the material is supplied to the inside of the cylinder 5.
On the other hand, the input to the injection molding machine 10 as the controlled object is a rotation amount (an integrated value of the speed N) of the servo motor 14. This rotation amount is a moving amount Ls of the screw 7 and is in proportion to a volume Vs of a moving portion of the screw 7 in the cylinder 5.
On the other hand, since the output of the injection molding machine can be expressed by the pressure p in the cylinder 5, the transfer function Ga of the injection molding machine 10 is given as the following equation.
Ga=p/Vsxe2x80x83xe2x80x83(1)
Where
Vs: changed volume (m3) of the cylinder 5,
p: pressure (kg/m3) in the cylinder 5.
Therefore, an overall block diagram of a control system in the electric injection molding machine is given as shown in FIG. 4(a). In FIG. 4(a),
Gp: dwell pressure control gain, GN: speed control gain, G1: current control gain, HI: current feedback gain, HN: speed feedback gain, Hp: pressure feedback gain, Kt, K1: constant, J: moment of inertia (kg/m2) of the servo motor 12, A: sectional area (m2) of the cylinder 5, S: Laplace operator, xcfx89: angular velocity of rotation (rad/s) of the servo motor 12, and Vs, p: mentioned above
Since the speed feedback gain HN is sufficiently larger than the gain G1 in a dotted line, the transfer function G0 in the dotted line can be assumed as 1/HN. Thus, the block diagram shown in FIG. 4(a) can be simplified as shown in FIG. 4(b).
In this FIG. 4(b), if the pressure feedback gain Hp is assumed as 1, the open-loop transfer function Gs from the pressure command Pr to the pressure feedback p can be given as the following equation.
Gs=p/Pr=(Gp/HN)xc3x97(Kc/S)xc3x97(p/Vs)xe2x80x83xe2x80x83(2)
Where
Kc: constant,
S: Laplace operator.
Here, the electric injection molding machine controls p/Vs in above Eq. (2) as a constant in the dwell pressure mode.
However, this p/Vs is not constant since such p/Vs is given by the relational curve between the pressure and the volume of the cylinder 5, as shown in FIG. 5.
The relational curve of FIG. 5 can be calculated as follows. That is, the ideal condition that the resin 3 in the cylinder 5 can be changed by the compression under the conditions that the temperature is constant and the entropy is constant is assumed and also the pressure of the resin 3 is assumed totally uniform in the cylinder 5.
Since the compression ratio "khgr" of the resin 3 in the cylinder 5 can be regarded as a constant according to such assumptions, the following equation can be satisfied.
"khgr"=xe2x88x92(1/Vs)xc3x97(dVs/dp)xe2x80x83xe2x80x83(3)
Where
Vs: volume (m3) in the cylinder 5,
p: pressure (kg/m3) in the cylinder 5.
Based on above Eq. (3), the volume Vs in the cylinder can be given as the following equation.
Vs=Vsoxc3x97xcex5xe2x88x92P"khgr"xe2x80x83xe2x80x83(4)
Where
Vso: initial value of the volume (m3).
Eq(4) gives the relational curve between the pressure and the volume in the cylinder 5, as shown in FIG. 5.
Accordingly, if the pressure is controlled based on the volume in the cylinder 5 (the moving amount Ls of the screw 7) in order to control the pressure in the cylinder 5, there is the following problem since the relationship between the volume and the pressure is nonlinear, as shown in FIG. 5.
In the dwell pressure mode, as shown in FIG. 5, under the dwell pressure control gain of the constant value that is multiplied by the pressure deviation, the volume V1 in the cylinder 5 is high if the pressure in the cylinder 5 has the low pressure value P1, and therefore P1/V1 has the small value. Thus, if it is tried to maintain the open-loop transfer function shown in FIG. 4(b) at the constant value, a set value of the dwell pressure control gain of the pressure amplifier 24 must be increased.
Under the dwell pressure control gain value set in this manner, the volume in the cylinder 5 is reduced like V2 if the pressure value of the cylinder 5 is increased to the pressure P2, for example, and therefore P2/V2 becomes large. Thus, the pressure value of the screw 7 is overshot at the pressure P2 against the pressure command Pr.
However, if the dwell pressure control gain value is set in a situation where the pressure in the cylinder 5 is high, there is a problem that the response to the pressure command Pr becomes slow when the pressure in the cylinder 5 is low.
The present invention has been made to overcome the above subjects, and it is an object of the present invention to provide an electric injection molding machine that can be controlled smoothly based on a pressure command.
An electric injection molding machine according to the present invention comprises an injection molding machine having a screw that can be moved in a cylinder; a pressure commanding means for generating a pressure command value applied to the screw; a pressure sensing means for sensing a pressure applied to the screw; a pressure deviating means for calculating a deviation value between the pressure command value of the pressure commanding means and a sensed value of the pressure sensing means; a pressure amplifying means for amplifying the deviation value of the pressure deviating means at a predetermined amplification factor; an amplification factor changing means for changing an amplification factor value of the pressure amplifying means based on a pressure value of the pressure sensing means; and a servo motor driven based on an output value of the pressure amplifying means to move the screw.
An electric injection molding machine according to the next present invention further comprises an amplification factor changing means for changing the amplification factor value of the pressure amplifying means based on the pressure command value of the pressure commanding means, in place of the amplification factor changing means for changing the amplification factor value of the pressure amplifying means based on the pressure value of the pressure sensing means.
In the electric injection molding machine according to the next present invention, the amplification factor changing means includes a storing means for receiving the pressure value of the pressure sensing means or the pressure command value of the pressure commanding means as address information and storing the amplification factor value that corresponds to the address information, a multiplying means for multiplying the amplification factor value read from the storing means and a deviation value of the pressure deviating means, and an amplifying means for amplifying an output of the multiplying means.
In the electric injection molding machine according to the next present invention, the amplification factor of the amplification factor changing means sets a ratio of the pressure value in the cylinder to a change amount of a volume of the cylinder to a substantially constant value.